What are the resonance structures for an acetate ion (CH2CO2-)?

Question

I understand the two resonance structures for the acetate ion are molecules with a double bond existing between the second carbon and one of the oxygens and the next resonance structure being the movement of that double bond to between the second carbon and the previously single bonded carbon, but why doesn't a resonance structure with all single bonds and a -1 formal charge on the second carbon and one of the oxygens and a +1 charge on the other oxygen exist?

Answer 1

Let me know if this answer doesn't make sense. So If you start out with the acetate ion with the first carbon bonded to the second carbon, which is double bonded to an O and single bonded to an O, you'll see you have a lone pair on the O that can form a bond with carbon 2. When you do that, you must push one of the bonds in the C=O up making an extra lone pair on the other O. These are the two structures you talk about in your question. You cant have your third one, because it would require that one of your oxygen atoms only has 6 electrons. Oxygen will NOT go without a complete octet while an adjacent carbon contains an extra lone pair of electrons. Oxygen is strongly electronegative and will pul those electrons back in a heartbeat. Thus, that structure is essentially nonexistant.